In recent years, with the advance of environmental technology, power generation devices (e.g., solar power generation devices) which pose less burden on the environment than conventional power generation methods have been actively developed. Concurrently with the development of power generation technology, development of power storage devices has also been underway.
As an example of power storage devices, a lithium secondary battery that is a secondary battery is given. A lithium secondary battery is widely spread because the lithium secondary battery has high energy density and is well suited for miniaturization. As an electrode material of a lithium secondary battery, a material in/from which lithium can be inserted/extracted is preferably used. Examples of the electrode material include a carbon-based material such as graphite and an alloy-based material such as silicon or silicon oxide. Among alloy-based materials, silicon-based active material is particularly expected to be a host material of lithium because the theoretical capacity of silicon is about ten times as high as that of graphite.
However, even in a lithium secondary battery in which a carbon-based material such as graphite or an alloy-based material such as silicon is used for a negative electrode active material, it is extremely difficult to extract all lithium introduced at charge from a positive electrode material such as LiFePO4, LiCoO2, or LiMn2O4 into the alloy-based material of the negative electrode active material is extracted at discharge. Accordingly, there is a problem in that a certain amount of lithium remains in the alloy-based material. In other words, there is a problem in that lithium is used for irreversible capacity, resulting in a decrease in discharge capacity of the secondary battery and a decrease in performance as a battery.
In order to solve such a problem, many researches have been conducted, and patent documents have reported on such researches (e.g., see Patent Documents 1 to 3). For example, Patent Document 1 discloses a lithium secondary battery formed in such a manner that a carbon material for a negative electrode is doped with a lithium-containing metal oxide for a positive electrode after the battery is assembled, and the positive electrode is doped with lithium provided in the battery. In this lithium secondary battery, a lithium metal and the positive electrode are connected through a resistive element, and the resistive element controls current flowing between the lithium metal and the positive electrode; thus, after the assembling of the battery, the positive electrode is doped with lithium by the electrochemical connection.
Patent Document 2 discloses a lithium secondary battery which includes a positive electrode formed with lithium manganese oxide serving as a positive electrode active material, and a negative electrode formed with a carbon material serving as a negative electrode active material. According to Patent Document 2, metal lithium is put in a battery can in advance; an electrolyte solution is added to cause short circuit between the metal lithium and the positive electrode, so that lithium corresponding to irreversible capacity of the negative electrode is introduced into the positive electrode; and then initial charge is performed between the positive electrode and the negative electrode. By this method, lithium ions transfer from the metal lithium to the positive electrode, and from the positive electrode to the negative electrode, so that the negative electrode is doped with the lithium ions. Consequently, the negative electrode can be doped with lithium, which corresponds to irreversible capacity.
Patent Document 3 discloses a negative electrode formed in such a manner that lithium insertion treatment is performed on a negative electrode material formed of silicon with an ion implantation apparatus in advance.